[UX][PS] Replace the current token browser in core with the Fast Token Browser implementation from contrib

Description of the need

I think https://github.com/backdrop-contrib/fast_token_browser is an improvement over the core version. It loads faster, has a more intuitive UI, and will be easier to make accessible to use via the keyboard (maybe it does already).

Proposed solution

Put FTB into core.

Additional information

FTB loads the tokens via ajax which makes it faster when there are a lot of tokens. And it also changes the way tokens are inserted. First the person selects the token - it shows up as selected. Then they click on / select the text field. It then gets inserted. Core has a clunkier approach which requires the unintuitive approach of selecting the field first then finding the token.

Also the modal window can be resized and moved.